Various multifrequency signal generators have been proposed and used heretofore in telephone dials employing electronic switching wherein the common switching functions are performed by solid-state circuit elements. The use of electronic switching is consistent with the current direction of telephone development which points toward a substantially fully integrated set employing monolithic circuit techniques. The electronic switching is thus replacing mechanical switches found in prior art dials and thereby exploiting modern circuit technology with its increased reliability and decreased cost.
In disconnecting the transmitter and muting the receiver during dialing, the mechanical common switches of the prior art dials simply make and break appropriate signal paths. In that the series resistance of the mechanical switch is essentially zero in the ON condition, these switches connect the transmitter and receiver to their respective matching loads with no loss of signal level or change of impedance.
The multifrequency signal generators in telephone dials having electronic switching generally duplicate the mechanical switching functions with active devices such as transistors. But these devices are lossy elements when in the ON condition (saturated mode) having a finite series resistance that becomes an important consideration on long loops with corresponding low loop current conditions. In an effort to overcome this disadvantage, it has been proposed that the ON resistance be minimized through enlargement of the area of the switching transistor on the integrated circuit. This, however, would make the common switch a disproportionately large area of the integrated circuit without further benefit other than the reduced resistance. In fact, there is always some minimal irreducible loss associated with the saturated transistor switch. Moreover, the transistor switch has a dc voltage drop that reduces the current in the transmitter which, in turn, means lower output signal since the transmitter is a variable resistance element.